Electroless metal plating is a known method for depositing a metal layer on a substrate surface. Electroless metal plating is used in various industries, including decorative plating and the manufacture of electronic components. Electroless metal plating is also widely used in the formation of electrical circuits on printed circuit boards. A catalyst must adhere to the surface in advance when conducting electroless plating on the surface of a nonconductive article. Palladium-tin colloid catalysts have been used as the catalyst in the past. Palladium-tin colloid catalysts are made by mixing tin(II) chloride and palladium chloride in an acidic solution.
However, palladium-tin colloid catalysts tend to aggregate readily. Since the aggregated colloid does not adhere uniformly to the surface of the article to be plated, the amount of plating deposited in the vicinity of the colloid becomes far greater, and metal deposits unevenly on the article during electroless plating. Uneven electroless plating deposition is said to be related to lowered adhesiveness between the plating film and substrate and lowered insulating property. Requests based on environmental issues in recent years have also led to demands for the development of tin-free catalysts.
So-called palladium ion catalysts in which palladium ion is stabilized by a complexing agent are being developed as substitutes for palladium-tin colloid catalysts. The use of amine compounds as complexing agents is known to not only form complexes of the palladium ion but also to heighten the catalyst-imparting property. For example, JP Kokai 61-15983 discloses a catalyst for electroless plating containing complexes obtained by dissolving at least one compound of palladium(II), silver(I), copper(I), copper(II), and nickel(II) in an amide JP Kokai 2007-138218 discloses a concentrated catalyst solution for electroless plating containing a divalent palladium compound and an amine complexing agent, discloses oxalic acid, tartaric acid, acetic acid, citric acid, phthalic acid, 2-(N-morpholino)ethanesulfonic acid, and the like as examples of buffers for pH adjustment, and describes concrete examples of buffer solution using boric acid. 2-(N-morpholino)ethanesulfonic acid is known as a compound that exhibits a buffering effect in the acidic region of pH 5.5-7.0. U.S. Pat. No. 5,503,877 discloses complex compounds containing metal having at least one organic ligand that are complex compounds in which the complex is present in the form of an oligomer or polymer. Amine compounds are given as examples of the organic ligand, and borates, carbonates, phosphates, and acetates are given as examples of buffers. The superiority of using boric acid as the buffer is specifically stated.
However, the boric acid used in these references is considered to be an environmentally hazardous substance and further restriction of its use is expected in the future. Substitutes for boric acid are therefore being sought. The present inventors also determined as based on their research that when boric acid is used as a buffer, the solution is stable and the catalyst performance is good in strongly alkaline region of pH 11 and above, but problems such as precipitation of palladium and lowered catalyst-imparting capability due to variations in complexing power arise when used in the weakly alkaline region. Consequently, it was not possible to impart catalyst adequately to substrates that employ polyimide resins and the like that deteriorate readily upon contact with highly alkaline solutions.